Gene Arrangement and Chiasmata 



therefore, relatively less frequent among the 

 adults scored for crossovers. It should be 

 noted also that zygotes destined to become 

 either miniature or cut are also discriminated 

 against more than are zygotes destined to 

 produce the wild -type. Whenever pheno- 

 types are scored after some long develop- 



+ m 

 ct + 



9 



- .-/ 



129 



ANY 



d 



FIGURE 17-1. Crossover rate for 

 two X-linked genes in Drosophila. 



Sons: + m / Y 42.5 % 



ct + / Y 42.5 % 



+ + / Y 7.5 % 



ct m / Y 7.5 % 



^^=^. 



mental period, much of the error due to 

 differential viability may be avoided by pro- 

 viding optimal culture conditions. Another 

 way to avoid most of this kind of error is to 

 obtain progeny carrying the chromosome to 

 be scored for crossovers, in which the homolo- 

 gous chromosome contains the normal alleles 

 of all genes under crossover test. Since such 

 progeny are phenotypically normal they will 

 all have approximately the same viability and 

 can be scored as to chromosome type from 

 the offspring each produces when individually 

 test crossed. Thus, for example, the female 

 in Figure 17-1 is crossed to wild-type males 



and the daughters (all phenotypically normal) 

 are individually mated to any male. Daugh- 

 ters which carry, in addition to + + on one 

 X obtained from their father, a homologous 

 X of one of the following types, + m, ct -\-, 

 -|- -|-, ctm, will produce sons, of which, 

 respectively, some are miniature but none 

 cut, some cut but none miniature, all normal 

 and some miniature and cut. In this way the 

 generation being tested for crossover rate is 

 protected from differential viability and its 

 genotypes are detected in the generation fol- 

 lowing. For some purposes the extra labor 

 entailed by the use of this method is justified. 



